The AutoPulse® chest compression device is used to provide chest compressions during the course of CPR in reviving a cardiac arrest victim. The AutoPulse® provides compressions according to a predetermined compression waveform which is optimized for a large variety of potential victims. We have previously proposed feedback control, based on sensed biological parameters, to alter the compression waveform applied by the chest compression device. The biological parameters proposed, including end-tidal CO2 and blood oxygen levels, are readily measured with non-invasive devices.
The operation of chest compression devices can be improved with the use of more fundamental biological parameters, such as aortic blood flow volume, the aortic pulse pressure waveform, and other blood vessel parameters, as feedback for control of the chest compression device. Depending on the value of aortic blood flow volume and blood vessel parameters, the compression waveform provided by the chest compression device may be varied. The compression waveform may be varied from patient to patient, depending on the value of aortic blood flow volume and/or blood vessel parameters measured before or at the commencement of chest compressions. The compression waveform may be varied during the course of CPR chest compression on a single patient, depending on the value of aortic blood flow volume and/or blood vessel parameters measured over the course of resuscitation efforts and chest compressions. Chest compression waveform characteristics such as compression depth, compression rate, compression rise time, compression hold time, and release velocity can be varied to optimize compression induced blood flow in the cardiac arrest victim.
Adjunct therapies, especially the administration of epinephrine, can be implemented, modified or avoided based on information gleaned from the biological parameters, such as arterial stiffness and/or pulse wave velocity.
A number of terms relating to blood flow parameters are used in the art, including the following:
The pulse pressure waveform is a depiction of pressure versus time in a particular blood vessel.
SPTI refers to the systolic pressure-time integral, which is the area under the central aortic pressure wave curve during the systole portion of a heartbeat (when the left ventricle is contracting). SPTI is also referred to as left ventricular load, or LV load. Systole is that portion of the heartbeat starting at the closure of the atrioventricular (cuspid) valves and ending with the closure of the aortic valve.
DPTI refers to the diastolic pressure-time integral, which is the area under the central aortic pressure wave curve during the diastole portion of a heartbeat (when the heart left ventricle is relaxing). Diastole is that portion of the heartbeat in which the heart is relaxing, starting with closure of the aortic valve and ending with the subsequent closure of the atrioventricular valves.
Arterial Compliance, a measure of the stiffness, refers to the mechanical characteristic of blood vessels throughout the body. If refers to the ability or inability of blood vessels to elastically expand in response to pulsatile flow. It is quantified in terms of ml/mm Hg (the change in volume due to a given change in pressure). Elastance is a reciprocal concept, and refers to the tendency of blood vessels to recoil after distension. In relation to the aorta, aortic compliance/elastance affects the ability of the aorta to expand and contract during and after contraction of the heart which forces blood from the left ventricle.
The aortic pulse pressure waveform can be determined non-invasively, based on peripheral pulse waveforms obtained with sensors mounted on the patient. Sensors can measure pressure and/or velocity at superficial locations of the radial artery, brachial artery, carotid and/or femoral artery. Various known models and “transfer functions” can be used to determine the aortic pressure wave from pressure waves measurements at peripheral locations such as the radial artery, brachial artery, carotid and/or femoral artery. See Chen, et al., Estimation of Central Aortic Pressure Waveform by Mathematical Transformation of Radial Tonometry Pressure, 95 Circulation 1827 (1997). The transfer function used for this estimate may be generalized, in the sense that the same generally applicable and sufficiently reliable transfer function is used to determine the aortic pressure wave for all patients. The transfer function can be different for known significantly different subpopulations, so that one transformation is applicable and sufficiently reliable for one group (men, for example) while a different transformation is applicable and sufficiently reliable for another group (women, for example). The transfer function can be individualized, such that, for each individual patient, a different transfer function is determined, and then used to estimate the aortic pressure wave from peripheral pressure waves. Use of non-invasive measurements to estimate aortic pressure wave allows for control of a chest compression device based on the pressure waveform in the field. (Waveforms obtained by invasive pressure sensors in the aorta might also be used in hospital, where it is more appropriate to install devices in the aorta of a patient).
Pulse wave velocity is used as a measure of arterial stiffness. It is defined as the velocity at which a pressure wave, travelling from the proximal aorta, travels to peripheral cites such as the superficially accessible portions of the carotid, brachial, radial or femoral arteries.
Pulse transit time is defined as the time it takes for a pulse waveform to travel from one location to another in the body. For example, the pulse transit time may be specified as the time it takes for a peak of the pulse pressure to travel from a proximal location to a more distal location in the arm, or from the carotid artery in the neck to the radial artery at the wrist. In some references, pulse transit time (PTT) is defined as the time it takes for the arterial pulse pressure wave, starting from the aortic valve, to reach a peripheral site. Pulse transit time is dependent on the resistance to flow presented by the peripheral blood vessels. High peripheral resistance is beneficial during CPR, because it limits blood flow to the peripheral blood vessels and thus forces any blood flow induced by compressions to the heart and brain.
Various values of these parameters have been associated with cardiovascular disease and risk of heart attack and stroke. They may be valuable in predicting the risk of future course of cardiovascular disease. These parameters have not been used as feedback for modification of resuscitation efforts for a patient in cardiac arrest. During sudden cardiac arrest and CPR chest compressions, some of the parameters become meaningless, while some parameters provide useful information pertaining to the course of CPR compressions and resuscitation. Some of the parameters, or related parameters, used for diagnosis can be used as feedback for control of CPR compression devices, while some related parameters defined below, which are meaningful solely in relation to CPR compressions, can be used as feedback for control of CPR compression devices.